1. Field of the Invention
This invention relates to gas turbine engines and more particularly to apparatus for cooling the case and the nozzle guide vanes in the turbine section of the engine.
2. Description of the Prior Art
A limiting factor in many turbine engine designs is the maximum temperature of the working medium gases that can be tolerated at the inlet to the turbine. A variety of techniques is used to increase the allowable inlet temperature including the cooling of the first few sets of nozzle guide vanes and rotor blades. Such cooling is commonly accomplished with air bled from the compressor and transferred to a local area to be cooled through suitable conduit means. The cooling air is at a pressure which is sufficiently high to permit the air to flow into the local area of the turbine without auxiliary pumping and at a temperature sufficient to provide the required cooling.
Impingement cooling is one of the more effective techniques used in cooling the turbine. In this type of cooling relatively high pressure air is passed through a multiplicity of orifices in a plate which is adjacent to the surface to be cooled causing jets of air to impinge upon local areas of the surface. The cooling rate in any local area is higher than that obtainable with conventional convective cooling thereby permitting exposure of the cooled components to higher gas temperatures without adversely effecting their durability.
Sufficiently high impingement velocities are obtainable with pressure differentials of the cooling air across the plate which are typically within the range of 20 to 70 pounds per square inch. Because of the substantial pressure differential a continuing problem in such constructions is the premature leakage of cooling air from the supply conduit means before the air can be discharged against the part to be cooled.
Considerable technical effort has been directed to the design of coolable components which minimize the potential for cooling air leakage. In U.S. Pat. No. 3,362,681 to Smuland apparatus which isolates the cooling flow to the nozzle guide vanes in the turbine section of an engine is shown. An arcuate plenum chamber is formed at the base of a plurality of integrally formed guide vanes. Cooling air supplied to the chamber is flowed to each of the vanes in the unit to cool the respective vane. The substantial premature leakage of cooling air between platforms of adjacent vanes is eliminated by the integrally formed construction of Smuland. In an engine, however, the vanes are exposed to extremely hot local gases which cause the blades to wear and ultimately require that the blades be periodically replaced. When this happens the integrally formed construction is unattractive as compared to a vane construction which permits the replacement of each vane individually according to local wear and deterioration.
In order to take advantage of the maintenance features of the individual vane construction, means must be devised to prevent the premature leakage of cooling air between the adjacent vanes. Numerous attempts have been made in the past to establish a mechanical seal between the vanes at that location but they have been only partially effective in reducing the amount of leakage. The amount of cooling flow lost across the mechanical seal increases substantially in proportion to the pressure differential between the cooling air supply and the local working medium gases. It is apparent that where impingement cooling techniques are utilized this pressure differential will be great and the associated losses will be significant.
Another limiting factor in many turbines is the radial clearance between the tips of the rotor blades and the opposing outer air seal which is supported by the turbine case. The clearance is made as small as possible to reduce the leakage of working medium gases around the tips from the pressure side to the suction side of each blade, but yet large enough to accommodate the thermal growth of the rotor. Because the rotor responds much faster to increased working medium temperatures than the case which supports the seal, a relatively large initial clearance is provided to prevent the destructive impact of the blade tips on the seal. The minimum clearance between the tips and the seal occurs when the rotor and the blades mounted thereon have expanded thermally ahead of the case. Once the case also responds, the clearance becomes increased and substantial leakage occurs.
In many engines cooling air is flowed through chambers adjacent to the case to limit the thermal response of the case and, accordingly, to minimize the clearance under steady state conditions. The chambers are positioned radially between the flow path of the working medium gases and the case and, in most embodiments, contain cooling air at a pressure greater than the local pressure of the medium gases. In order to make judicious use of the cooling air, radial seals between the cooling air and the medium gases must be maintained.
Overall engine performance can be increased by reducing the amount of cooling flow. Accordingly, continuing efforts are underway to treat the problem of cooling air leakage in an effective manner while maintaining or increasing the standards of component durability.